1. Field of the Invention
The present invention relates to a detection circuit for detecting signal amplitude.
2. Related Background of the Invention
An optical transmitter module used for an optical communication is configured by a light source such as a laser diode, an optical modulator such as an electro-absorption modulator (EAM) or a Mach-Zehnder modulator (MZM), and a modulator driving circuit. In addition, a detection circuit for detecting an amplitude of a driving signal to drive the optical modulator is sometimes added to the modulator driving circuit. With respect to the detection circuit used in the optical transmitter module for high-speed communication of 32 Gbps or 40 Gbps, high-speed detection is required because a communication signal has a high frequency component up to approximately 10 GHz. In addition, in a multi-level modulation method such as pulse amplitude modulation 4 (PAM4) or 16 quadrature amplitude modulation (16 QAM), a more precise detection is required as compared with a conventional on-off-keying method.
As an example of the detection circuit, a configuration described in patent document 1: Japanese Unexamined Patent Application Publication No. 2012-215662 described below is known. The amplitude detection circuit includes a peak detection circuit and an averaging circuit, and an amplitude is found as the difference between outputs thereof. The peak detection circuit is mainly configured by an operational amplifier and a peak hold circuit, and when a frequency band of a detection circuit is set to be 10 GHz or more, the detection circuit needs an operational amplifier operational with a frequency greater than 10 GHz. In addition, since an input offset value of the operational amplifier causes an error of a peak detection value, the gain of the operational amplifier is required to be maintained to be high up to a high frequency in order to decrease the error. Here, if an operation speed of the modulator driving circuit is 40 Gbps, an frequency band thereof is approximately 30 GHz. If an operational amplifier with a high gain at 10 GHz or more is used in the peak detection circuit using the same transistors, power consumption thereof increases to a large value of approximately 1/10 to ⅓ of the modulator driving circuit. Therefore, the peak detection circuit described in the patent document 1 described above is not appropriate for peak detection of a signal with a high frequency.
In addition, as another configuration of the amplitude detection circuit, the configuration described in non-patent document 1: “Daniel Kucharski, ‘Jitter Considerations in the Design of a 10-Gb/s Automatic Gain Control Amplifier’, IEEE Transactions of Microwave Theory and Techniques, Vol. 53, No. 2, February 2005” is known. In the configuration, since a peak detection circuit is configured only by a peak hold circuit, not by any operational amplifier, it is possible to handle a high frequency signal with low power consumption.
In the detection circuit described in the non-patent document 1 described above, an error of a detected amplitude is increased depending on a signal waveform. That is, a ratio of on-time when an input signal is close to a peak value of it with respect to off-time when the input signal is lower away from the peak value becomes small, and thus an error of the detected amplitude tends to be degraded.
Therefore, an object of the present invention is to solve such problems, and to provide a detection circuit for detecting signal amplitude which can accurately detect an amplitude of an input signal with a high frequency.